1. Field of the Invention
This invention relates to analog-to-digital converters. More particularly, this invention relates to a two-stage a-to-d converter wherein the first stage develops a set of higher-order bits of the digital output signal, and the second stage develops the remaining, lower-order bits.
2. Description of the Prior Art
With respect first to digital-to-analog converters, it has been known in the art to provide such converters having two successive stages, wherein the first stage resolves a set of higher-order bits of the input signal into a corresponding analog signal, and the second stage resolves the remaining, lower-order bits to produce a second analog signal to be added to the first analog signal. For example, Susset U.S. Pat. No. 3,997,892 shows such a converter wherein the first stage is a non-linear resistor-string d-to-a converter employing resistors of unequal values to produce a first voltage corresponding to a set of higher-order input bits, and the second stage is a linear resistor-string d-to-a converter arranged to produce a second voltage corresponding to the lower-order bits. Resistorstring converters have the desirable property of assured monotonicity, in that for each increase (decrease) in the digital input, there is a corresponding increase (decrease) in the analog output. In a two-stage digital-to-analog converter as shown by Susset, wherein the first stage is a non-linear converter, the voltage developed by that converter varies non-uniformly with uniform changes in the first stage digital input signal. Thus the voltage across the selected resistor is applied to the second stage converter so that it can develop a voltage intermediate the particular two voltages at the end terminals of the resistor selected by the first-stage digital signal.
With respect now to analog-to-digital converters, many different kinds have been in use for considerable time. In recent years, it has become important to provide such converters with extended resolution, e.g. up to sixteen or so bits, while yet assuring accurate performance. It is an object of this invention to provide such an improved analog-to-digital converter.